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The present invention relates to optical sensors, and in particular to highly sensitive, integrated microcavity-waveguide sensor.
During the past few years, a substantial amount of research has been performed in the field of optical microcavity physics, in order to develop high cavity-Q optical microcavity resonators. In general, resonant cavities that can store and recirculate electromagnetic energy at optical frequencies have many useful applications, including high-precision spectroscopy, signal processing, sensing, and filtering. Many difficulties present themselves when conventional planar technology, i.e. etching, is used in order to fabricate high quality optical resonators, because the surfaces must show deviations of less than about a few nanometers. Optical microcavity resonators, on the other hand, can have quality factors that are several orders of magnitude better than typical surface etched optical resonators, because these microcavities can be shaped by natural surface tension forces during a liquid state fabrication. The result is a clean, smooth silica surface with low optical loss and negligible scattering. These microcavities are inexpensive, simple to fabricate, and are compatible with integrated optics.
Optical microcavity resonators have quality factors (Qs) that are higher by several orders of magnitude, as compared to other electromagnetic devices. Measured Qs as large at 1010 have been reported, whereas commercially available devices typically have Qs ranging from about 105 to about 107. The high-Q resonances encountered in these microcavities are due to optical whispering-gallery-modes (WGM) that are supported within the microcavities.
As a result of their small size and high cavity Q, interest has recently grown in potential applications of microcavities to fields such as electro-optics, microlaser development, measurement science, and spectroscopy. By making use of these high Q values, microspheric cavities have the potential to provide unprecedented performance in numerous applications. For example, these microspheric cavities may be useful in applications that call for ultra-narrow linewidths, long energy decay times, large energy densities, and fine sensing of environmental changes, to cite just a few examples.
In order for the potential of microcavity-based devices to be realized, it is necessary to couple light selectively and efficiently into the microspheres. Since the ultra-high Q values of microcavities are the result of energy that is tightly bound inside the cavity, optical energy must be coupled in and out of the high Q cavities, without negatively affecting the Q. Further, the stable integration of the microcavities with the input and output light coupling media should be achieved. Also, controlling the excitation of resonant modes within these microcavities is necessary for proper device performance, but presents a challenge for conventional waveguides.
Typically, good overall performance is gained by accessing the evanescent field in a waveguide. Also, only waveguide structures provide easy alignment and discrete, clearly defined ports. Because of cavity and waveguide mode leakage into the substrate and into the modes within the fiber cladding, power extraction from the input optical radiation has proved to be inefficient for conventional planar waveguides, however.
U.S. patent application Ser. No. 09/893,954 discloses a highly efficient and robust mechanism for coupling optical microcavity whispering-gallery modes into integrated optical waveguide chips. SPARROW (Stripline Pedestal Antiresonant Reflecting Waveguides) are used to achieve vertical confinement and substrate isolation through a highly reflective stack of alternating high and low refractive index dielectric layers. Q-values of over 109, and coupling efficiencies of over 99% have been observed.
Because of the ability of SPARROW waveguide chips to excite resonant modes having unprecedentedly high Q-values in optical microcavities, it is desirable to implement SPARROW waveguide chips in sensing applications, so as to increase the resolution and dynamic range in these applications.
The present invention is directed to the implementation of a waveguide-coupled optical microcavity resonator for sensing applications. In particular, a SPARROW (Stripline Pedestal Antiresonant Reflective Optical Waveguide) optical chip structure is used to evanescently couple light into an optical microcavity at very high efficiencies, approaching 100%. An input, for example an external force or a change in an environmental condition such as temperature, causes the microcavity to move, and causes a change in the coupling geometry between the microcavity and the optical waveguide. In one embodiment, the change in coupling geometry is caused by a displacement of the microcavity in response to the inertial input, the displacement resulting in a change in the coupling gap between the microcavity and the waveguide. Using a sensor constructed in accordance with the present invention, a resolution limit of about 1017m and a dynamic range of about 1010 can be reached for the sensing of acceleration, representing an improvement over prior art accelerators of several orders of magnitude.
An optical resonator sensor, constructed in accordance with the present invention, includes a substrate, a SPARROW optical waveguide disposed on the substrate, an optical microcavity, and a flexure. The optical microcavity is a fused silica microcavity, capable of supporting high Q-factor whispering-gallery-modes (WGM). Photons within these modes are strongly confined slightly below the surface of the microsphere, due to repeated total internal reflection, thus resulting in very long cavity lifetimes and photon path lengths. Cavity Qs as high as 1010 have been reported.
The SPARROW optical waveguide includes a multi-layer dielectric stack disposed on the substrate, and a waveguide core. The dielectric stack includes alternating high and low refractive index dielectric layers, and is highly reflective. The reflectivity of the dielectric is sufficient to isolate the optical modes in the waveguide. The waveguide core is substantially planar, and is disposed on the dielectric stack. The waveguide core extends along an axis, parallel to the waveguide plane, from an input end to an output end. The waveguide core is adapted for transmitting light incident on the input end to the output end.
An optical microcavity is constructed and arranged so as to optically interact with light incident on the input end of the optical waveguide core. In one embodiment, the microcavity may be disposed along a sensing axis, perpendicular to the waveguide plane. A flexure has a first end coupled to the substrate, and a second end coupled to the optical microcavity. The flexure is responsive to an input, such as an external force or acceleration, to cause a change in the coupling geometry of the optical microcavity along the sensing axis.
The readout response of the sensor to an input can be determined by measuring a variety of parameters, including but not limited to the coupling gap, the resonance linewidth of the microcavity, the coupling strength of the microcavity, and the resonant frequency of the microcavity. Because of the high Qs of the microcavities, the resolution and dynamic range of the sensor can be increased significantly. Sensor resolution of about 10xe2x88x9217 m and a dynamic range of about 1010 have been attained for acceleration measurements.